The invention relates to an optical telecommunication element which comprises an undulated optical fiber inside a cylindrical sheath. The undulated optical fiber is fixed at periodic locations on the inner wall of the cylindrical sheath.
Such an element is disclosed in U.S. Pat. No. 4,039,248. In the known element one optical fiber is used. The fiber extends sinusoidally or substantially sinusoidally inside the sheath. (FIGS. 3-6.) The optical fiber may be situated in one plane but may also be successively orientated in several planes which enclose acute angles relative to one another. (FIG. 1b.) The amplitude of the sinusoidal optical fiber corresponds to the inside diameter of the sheath. By using a sheath whose inner surface comprises internal protusions or by using discs provided in the sheath the optical fiber is fixed inside the sheath.
The optical fiber described in U.S. Pat. No. 4,039,248 has a much smaller coefficient of expansion than the sheath of synthetic resin. Temperature variations will therefore cause a compressive force or tensile (pulling) force to be exerted on the fiber so that at the locations where the fiber is fixed to the sheath wall, variation in the radius of curvature and micro-bends will occur. This results in very considerable signal losses, and there is also a fair chance that the fiber will develop micro-cracks and finally break. The known element has the additional disadvantage that positioning and, in particular, fixing of the fiber inside the sheath is rather complicated. The manufacturing process can easily give rise to errors so that this process is rather critical and consequently is expensive.